Natural gas is liquefied to facilitate its transportation. Prior to liquefaction, raw natural gas must generally be treated to remove components which can freeze and plug equipment during the formation and/or processing of liquefied natural gas (LNG). Thus, water, carbon dioxide and heavier hydrocarbon components containing 5 or more carbon atoms (C.sub.5+) are generally removed.
It has typically also been desirable to fractionate natural gas into its various hydrocarbon components. Ethane, propane and butane (C.sub.2 -C.sub.4) are commonly used as refrigerants for natural gas liquefaction in the so-called multicomponent or cascade refrigeration processes. Pentanes and heavier hydrocarbons generally have greater economic value as NGL's (natural gas liquids) for use in chemical feed stocks and gasoline. Fractionation processes typically involve cooling the natural gas to effect a partial condensation and feeding the partially condensed stream to a fractionation column commonly known as a scrub column. Methane is taken primarily in the overhead vapor and heavier components are removed primarily as a bottoms liquid. The bottoms are usually fractionated further into individual C.sub.2 -C.sub.4 components for makeup gas in the LNG refrigeration system (e.g. multicomponent or cascade) and/or in order to make a liquefied petroleum gas (LPG) product. Typically, the scrub column employs either an overhead condensate reflux or a butane wash.
In circumstances where removal of freezable hydrocarbons prior to natural gas liquefaction is the primary requirement, the prior art fails to recognize inefficiencies in scrubbing systems. For example, in liquid natural gas (LNG) plants employing liquid nitrogen as the primary refrigerant, or where C.sub.2 -C.sub.4 refrigerants are already available from other sources, C.sub.2 -C.sub.4 fractionation may be unnecessary. Or, if the feed gas is very lean, fractionation may not be economical. The prior art processes for pretreating natural gas prior to liquefaction are not well-suited for such circumstances, are not energy efficient and incur excessive capital equipment costs.
U.S. Pat. No. 4,012,212 to Kniel describes a process for liquefying a hydrocarbon gas under a pressure greater than the critical pressure thereof wherein the gas is expanded to below the critical pressure and fed to a first fractionator. The first fractionator removes the light components from the feed gas for subsequent liquefaction. The bottoms of the first column are fed to a second fractionator wherein a butane-rich stream is separated from the C.sub.5 and heavier hydrocarbons to provide a reflux liquid for the first fractionator.
U.S. Pat. No. 4,070,165 to Colton describes a pretreatment process for raw natural gas prior to liquefaction. After water and acid gas removal, the high pressure gas is expanded and scrubbed with a butane-rich liquid previously separated from the gas to remove heavy hydrocarbons. A scrubbing column separates the lighter components for subsequent liquefaction and the bottoms are fractionated into the major components and the butane-rich liquid.
U.S. Pat. No. to 4,430,103 to Gray et al. describes a process for the cryogenic recovery of LNG from natural gas. A natural gas stream predominating in methane and containing significant amounts of C.sub.2, C.sub.3, C.sub.4, and C.sub.5 and higher molecular weight hydrocarbons is cooled in a plurality of cooling stages to a temperature sufficient to produce at least one heavy component liquid phase. In one of the intermediate cooling stages, the liquid phase and a portion of the vapor phase are combined and fed to a column. The remaining portion of the vapor phase is further cooled and the liquid phase of these stages provides a reflux liquid for the column. The bottoms from the column are further fractionated to provide C.sub.2 and C.sub.3 makeup gas for the cooling stages and separate C.sub.5+ liquids.
U.S. Pat. No. 4,445,917 to Chiu describes a process for producing a purified natural gas from a raw gas feed containing methane and hydrocarbon impurities of C.sub.2 and heavier. The raw feed is cooled, distilled to remove impurities and purified such that the distillation reflux is supplied by a portion of a subcooled methane-rich liquid stream.
U.S. Pat. No. 3,817,046 to Aoki et al. describes a combination cooling system useful for the liquefaction of natural gas. The cooling system employs a multi-component cooling cycle coupled to an absorption refrigerant cycle and heat from turbine exhaust. A distillation column is used to remove heavy components which can freeze. The vapor phase removed from the column is cooled to provide condensate for reflux and the vapor portion is then liquefied.
U.S. Pat. No. 4,445,916 to Newton describes a process for liquefying natural gas in which heavier components are separated in a scrub column prior to liquefaction. The feed to the scrub column is intercooled against the methane-rich overhead from the column and expanded.
U.S. Pat. No. 3,440,828 to Pryor et al. describes a process for liquefying natural gas using cascade refrigeration. The raw gas is partially cooled using a propane refrigeration cycle and fed to a distillation column to remove hexane. The overhead vapor is cooled using an ethylene refrigeration cycle and a liquid phase produced provides a reflux for the distillation column. The vapor of the ethylene cooling cycle is cooled in a methane cycle then expanded and fed to a stripping column wherein the liquid feed is stripped of nitrogen.
U.S. Pat. No. 3,724,226 to Pachaly describes a process for the liquefaction of natural gas. The raw gas is cryogenically fractionated to remove the CO.sub.2 and C.sub.5+ hydrocarbons and the purified feedstock is cooled and liquefied under pressure. The overhead vapor of the fractionation column is partially condensed to provide a reflux.
U.S. Pat. No. 4,881,960 to Ranke et al. describes a process for scrubbing a hydrocarbon stream rich in C.sub.2+ with a physical scrubbing agent in a column to remove the C.sub.2+ components. The scrubbing agent is a C.sub.4+ bottoms product having a suitable composition.
U.S. Pat. No. 4,519,824 to Huebel describes a cryogenic process for separating methane from ethane and heavier hydrocarbons in which a high pressure gas feed is divided into two gas streams. The gas is cooled either before or after it is divided. The divided gas streams are selectively cooled, expanded and separated into vapor and condensate streams and fed to a fractionation column.
Other U.S. Patents of interest include U.S. Pat. No. 4,022,597 to Bacon; U.S. Pat. No. 3,702,541 to Randall et al.; U.S. Pat. No. 4,698,081 to Aghili; U.S. Pat. No. 4,597,788 to Apffel; and U.S. Pat. No. 4,596,588 to Cook.